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  <meta name="description" content="使用FileChannel读写文件，我们只需要关注缓冲区Buffer，只需要对Buffer进行读写操作。 我们关注的只是Buffer的读写， 当需要向文件中写入数据时，将数据写入Buffer，然后使用Channel读取Buffer中的内容写出到文件。对于Buffer，这是一个“写入——读取”的过程。 当需要从文件中读取数据时，使用channel读取文件的内容写入到Buffer，然后读取Buffer">
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          FileChannel读写文件与filp()等方法
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        <p>使用FileChannel读写文件，我们只需要关注缓冲区Buffer，只需要对Buffer进行读写操作。</p>
<p>我们关注的只是Buffer的读写，</p>
<p>当需要向文件中写入数据时，将数据写入Buffer，然后使用Channel读取Buffer中的内容写出到文件。对于Buffer，这是一个“写入——读取”的过程。</p>
<p>当需要从文件中读取数据时，使用channel读取文件的内容写入到Buffer，然后读取Buffer的数据。对于Buffer，这是一个“写入——读取”的过程。</p>
 <span id="more"></span>

<p>Buffer有三个重要的参数：position，capaticy，limit。position可以是记录当前位置的指针，读写将从position位置开始；capaticy是Buffer的最大容量；limit可以看作是当前Buffer的“实际长度”，在向Buffer写入数据时，limit应当与capaticy相同；在从Buffer读取数据时，limit应当指向当前存储的内容的末尾，也即是position指向的位置。</p>
<p>使用channel读写文件通常如下：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br></pre></td><td class="code"><pre><span class="line"><span class="type">Path</span> <span class="variable">path</span> <span class="operator">=</span> Paths.get(<span class="string">&quot;FileChannelTest&quot;</span>);</span><br><span class="line"><span class="keyword">if</span> (!Files.exists(path)) &#123;</span><br><span class="line">       Files.createFile(path);</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="type">FileChannel</span> <span class="variable">channelWrite</span> <span class="operator">=</span> FileChannel.open(path, StandardOpenOption.WRITE);</span><br><span class="line"></span><br><span class="line"><span class="type">ByteBuffer</span> <span class="variable">byteBuffer1</span> <span class="operator">=</span> ByteBuffer.allocate(<span class="number">1024</span>);</span><br><span class="line"></span><br><span class="line">byteBuffer1.clear();</span><br><span class="line">byteBuffer1.putChar(<span class="string">&#x27;a&#x27;</span>);</span><br><span class="line">byteBuffer1.putInt(<span class="number">123</span>);</span><br><span class="line"></span><br><span class="line">byteBuffer1.flip();</span><br><span class="line">channelWrite.write(byteBuffer1);</span><br><span class="line"></span><br><span class="line">channelWrite.close();</span><br><span class="line"></span><br><span class="line"><span class="type">FileChannel</span> <span class="variable">channelRead</span> <span class="operator">=</span> FileChannel.open(path, StandardOpenOption.READ);</span><br><span class="line"></span><br><span class="line"><span class="type">ByteBuffer</span> <span class="variable">byteBuffer2</span> <span class="operator">=</span> ByteBuffer.allocate(<span class="number">1024</span>);</span><br><span class="line"></span><br><span class="line">byteBuffer2.clear();</span><br><span class="line">channelRead.read(byteBuffer2);</span><br><span class="line"></span><br><span class="line">byteBuffer2.flip();</span><br><span class="line">System.out.println(byteBuffer2.getChar());</span><br><span class="line">System.out.println(byteBuffer2.getInt());</span><br><span class="line"></span><br><span class="line">channelRead.close();</span><br></pre></td></tr></table></figure>

<p>需要特别的注意的是clear()和flip()方法。注释说起来绕口，直接对着代码看，</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">final</span> Buffer <span class="title function_">clear</span><span class="params">()</span> &#123;</span><br><span class="line">    position = <span class="number">0</span>;</span><br><span class="line">    limit = capacity;</span><br><span class="line">    mark = -<span class="number">1</span>;</span><br><span class="line">    <span class="keyword">return</span> <span class="built_in">this</span>;</span><br><span class="line">    &#125;</span><br></pre></td></tr></table></figure>

<p>clear()方法将指针pisition放回起点，limit指向缓冲区末尾边界，那么调用clear()之后，如果向Buffer写入数据，则从起点开始写，最大可以写入到末尾边界，覆盖原有内容。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">final</span> Buffer <span class="title function_">flip</span><span class="params">()</span> &#123;</span><br><span class="line">    limit = position;</span><br><span class="line">    position = <span class="number">0</span>;</span><br><span class="line">    mark = -<span class="number">1</span>;</span><br><span class="line">    <span class="keyword">return</span> <span class="built_in">this</span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>flip()方法将limit设置为当前指针位置，指针返回缓冲区首部，那么调用flip()之后，如果从Buffer读取数据，将读到0~limit之间的内容，也就是上次写入到Buffer中的内容。</p>
<p>所以在向Buffer写入数据时，如果要覆盖，应该先调用clear()；从Buffer读取内容时，应当先调用flip()。</p>

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